Salt bath for electrolytic cleaning of metals



3,096,261 SALT BATH FOR ELECTROLYTIC CLEANING F METALS Matthew Mekjean, Niagara Falls, N.Y., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed May 25, 1959, Ser. No. 815,270 11 Claims. (Cl. 204-141) The present invention relates to a salt bath for the electrolytic cleaning of metals. More particularly, the present invention resides in a new and improved molten alkali metal hydroxide based salt bath for the electrolytic cleaning of metals.

The commercially available alkali metal hydroxide based salt baths for the electrolytic cleaning of metals operate unsatisfactorily after extended idling periods. The metal workpieces which result from the use of these baths after such shutdowns appear a dark gray or brown color and have a dirty unattractive appearance. The bath must be primed electrolytically before workpieces can be acceptably descaled and cleaned. This priming may be accomplished by using the bath electrolytically for at least one hour, i.e., at least one hour of valuable production time must be consumed during which time the workpieces are rejected due to a dark gray or brown color and a nonmetallic, dirty unattractive appearance. Furthermore, heretofore, when the bath is in the primed condition, the workpieces emerge from the bath relatively clean, i.e., light metallic or slivery gray, but with stains varying in color from straw yellow to dark brown in a patternless mottled effect.

It is therefore, an object of the present invention to provide an alkali metal hydroxide based salt bath for the electrolytic cleaning of metals, wherein said bath need not be primed electrolytically after periods of inactivity.

It is a further object of the present invention to provide a chemical means for priming an alkali metal hydroxide based salt bath for the electrolyte cleaning of metals, said chemical means being safe, inexpensive and expeditious.

It is a still further object of the present invention to provide an alkali metal hydroxide based salt bath for the electrolytic cleaning of metals, wherein when said bath is primed, it is not subject to objectionable staining of workpieces.

Further objects of the present invention will appear hereinafter.

In accordance with the present invention, I have found a novel, substantially anhydrous, molten alkali metal hydroxide based salt for the electrolytic cleaning of metals, preferably ferrous metals, cuprous metals, nickelous metals, chromium metals and alloys thereof, comprising at least fifty percent by weight of an alkali metal hydroxide and at least 0.05 percent by weight of an additive selected from the group consisting of elemental phosphorus, elemental iron, a phosphide, a phosphite and mixtures thereof.

By employing the salt bath composition in accordance with the process of the present invention, I have obviated the necessity for expensive and time consuming electrolytic priming of the salt bath after periods of inactivity. In addition the process of the present invention is safe, inexpensive and expeditious. Furthermore, after the bath is primed, metals may be eifectively cleaned without objectionable straining of workpieces. Further advantages of the composition of the present invention will be apparent to those skilled in the art upon reading the present specification.

The alkali hydroxides which may be used include sodium, potassium, cesium, lithium, rubidium and the other atent 3,096,261 Patented July 2, 1963 ice alkali metals, as well as mixtures thereof. Because of avaliability and lower cost, however, sodium hydroxide is preferred in this composition, but this is not to be taken as limiting the scope of the invention in any way. The alkali metal hydroxide must be present in at least fifty percent by weight of the total composition, with the preferred composition containing at least eighty percent by weight. When larger amounts of alkali metal hydroxides are employed (e.g., ninety-five percent or greater), the only other constituent may be the additives of the present invention. When smaller amounts of alkali metal hydroxide are employed the remainder of the composition in addition to the additives of the present invention may be any of a number of special additives to accomplish special results, said special additives being known to the art, and some of which being enumerated hereinafter.

Technical grade caustic soda is normally preferred since it is commercially available at a relatively low cost. Hence, when technical grade caustic soda is employed, small amounts of impurities normally associated therewith are present in the bath, i.e., around three percent cumulatively by weight of sodium chloride, sodium carbonate, etc. based on the total composition. It will be noted that in use the molten salt bath slowly absorbs carbon dioxide from the atmosphere and converts some of the alkali hydroxide to alkali carbonate. Another source of carbon dioxide in the proximity of the bath would be the exhausts of gas or oil-fired heating tubes for maintaining the salt in the molten state. This conversion of alkali hydroxide to alkali carbonate in no way adversely affects the utility of the salt bath, in fact, carbonate below thirty percent increases the fluidity of the salt due to a lowering of the melting point of the mixture.

The additives of the present invention should be present in at least 0.05 percent by weight based on the total composition. The upper limit is determined by the solubility of the additives in the caustic bath. It will be noted that the additives should be added slowly to the caustic bath due to the fact that the additives of the present invention tend to react with available oxygen in the bath. A preferred amount of additives has been found to be around 0.5 percent by weight based on the total composition; however, larger amounts may be profitably employed.

The additives of the present invention are gettering agents selected from the group consisting of elemental phosphorus, elemental iron, an inorganic phosphide, an inorganic phosphite, or mixtures thereof. Typical metallic phosphides that may be employed include ferrophosphorus (iron phosphide), aluminum phosphide, calcium phosphide, arsenous phosphide, boron phosphide, cobalt phosphide, eupric phosphide, cuprous phosphide, molybdenum phosphide, stannic phosphide, etc. Typical metallis phosphites include sodium hypophosphite, sodium phosphite, calcium phosphite, calcium hypophosphite, cobaltous orthophosphite, lead phosphite, magnesium orthophosphite, potassium orthophosphite, potassium hypophosphite, etc. The preferred additives of the present invention are the phosphides, and in particular ferrophosphorus baceuse of commercial availability and low cost.

The necessity for priming will recure after an extended idling period. The length of time after which priming is required will vary depending on the size of the bath, temperature of the bath, the type of metals descaled prior to idling, composition of the bath, etc.

The special additives which may be included to achieve more desirable results for specific applications include one or more of the following: phosphates, chlorides, carbonates, fluorides, silicates, aluminates, stannates, borates, sulfates, sulfites, molybdates, fluosilicates and fluoborates.

3 It is preferred to use the alkali metal additives since this corresponds to the bath; however, others can be used. The use of these additives would be apparent to one skilled in the art. It is to be understood that the bath of the present invention is predominately non-oxidizing and non-reducing in character.

The temperatures at which the molten salt may be economically used can be varied over a range between about 650 degrees Fahrenheit to about 1250 degrees Fahrenheit, although the preferred operating temperature would be in the range of from about 800 degrees Fahrenheit to about 1000 degrees Fahrenheit. The bath may be kept in the molten state by the conventional methods known to those skilled in the art. For example, small furnaces might be economically controlled by electric heating. Larger commercially operated baths, which would have high heat dragon-t, might be more economically heated by gas fired immersion tubes. Other heating methods which appear convenient may be utilized.

In all cases the specific volume of the metal being treated and the thickness of the scale on the workpiece will control the time of the descaling cycle. Another variable which will effect the length of time of immersion is the temperature at which the bath is maintained. The current density or amperes per unit area of the total surface of the workpiece will also determine the length of the processing cycle.

The molten salt is used electrolytically to descale the various metals mentioned herein and their alloys when the workpiece is made the cathode. By making the workpiece the anode in the circuit, surface carbon, graphite and silicon can be removed, and an oxide is produced on the surfaces of the workpiece. This oxide is a thin, uniform protective film which is somewhat porous, so that the film may be sealed and finished by standard methods, such as linseed oil, lacquer, paint, wax, etc. The film may also be advantageously used as a lubricant in wire drawing. If the workpiece is to be oxide-free, the current is reversed, the oxide is reduced, and the workpiece is removed in the metallic state, bright and unstained, usually without the need of any subsequent treatment or acid pickling.

The following examples are given to illustrate the present invention and are not to be taken as limitative.

In the following examples the molten salt bath employed had the following composition, wherein amount of ingredients is given in percent by weight:

Sodium hydroxide 92.0 Sodium phosphate 5 .0 Sodium chloride 1.5 Sodiumcarbonate 1.5

Example ].-Descaling Cast Iron in an Unprimed Idled Bath A molten salt bath was used for the electrolytic descaling of ferrous castings for three days, operating at a temperature of 900 degrees Fahrenheit. The bath was then idled for a period of 68 hours at a temperature of 900 degrees Fahrenheit. Thereafter, a small, gray, badly sealed ferrous casting was made the cathode with the container as the anode and the temperature maintained at 900 degrees Fahrenheit. Direct current was supplied by a selenium rectifier. Twenty-five amperes were passed through the system for ten minutes at three volts. The casting was then removed from the pot and quenched in water. The casting was dark brown, although it was fully descaled.

Example 2.Descaling Cast Iron in an Electrolytically Primed Bath Small, gray, badly scaled ferrous castings were continuously descaled electrolytically in the bath of Example 1 in a manner after Example 1. After approximately forty minutes the castings emerged with a dark gray, but

l metallic appearance. After approximately one hour, the castings emerged descaled, bright and clean, except for a yellow, mottled stain. All samples subsequently processed continued to emerge descaled, bright and clean, except for a yellow, mottled stain, as long as there were no extended idling periods.

Example 3.-Descaling Cast Iron in a F errophosphorus Primed Bath Example 4.Descaling N ickeI-Steel in a Ferrophosphorus Primed Bath Example 3 was repeated electrolytically descaling a nickel-steel panel 3.5 by 6.0 inches of the proximate composition of 50 percent nickel and 50 percent iron, covered with a hard, lustrous, black oxide scale. The panel emerged completely descaled and silvery white, with no evidence of the mottled stain of Example 2. Subsequent samples continued to be descaled with similar results.

Example 5 .-Descaling Brass in a F errophosphorus Primed Bath Example 3 was repated electrolytically descaling a one-inch brass valve covered with a non-uniform oxide ranging in color from dark gray to black. The valve was completely descaled, both on the exterior and interior, uniformly clean and unstained and exhibiting the typical copper-red color of the brass. Subsequent samples continued to be descaled with similar results.

Example 6 .-Descaling Cast Iron, Cuprous and N ickelous Alloys in an Aluminum Phosphite Primed Bath Example 3 was repeated, except one-tenth of one percent of aluminum phosphite (AlP) was employed instead of ferrophosphorus. Similar results were obtained. Badly scaled cuprous and nickelous alloys were also descaled electrolytically, all with similar results.

Example 7.Descaling Cast Iron, Nickel-Steel and Brass in a Calcium Phosphite Primed Bath Example 3 was repeated, except one-tenth of one percent of calcium pho-sphite (Ca P was employed instead of ferrophosphorus. Similar results were obtained. Badly scaled brass and nickel-steel were also descaled electrolytically, all with similar results.

Example 8.Descaling Cast Iron in an Elemental Phosphorus Primed Bath Example 3 was repeated except the bath was maintained at 800 degrees Fahrenheit and one-tenth of one percent of elemental, powdered, red phosphorus (P was employed instead of ferrophosphorus. Similar results were obtained.

Example 9.Descaling Cast Iron in a Sodium H ypophosphite Primed Bath Example 3 was repeated except the bath was maintained at 800 degrees Fahrenheit and one-tenth of one percent of sodium hypophosphite (NaI-l PO l-l O) was employed instead of ferrophosphorus. Similar results were obtained.

Example 10.Descaling Cast Iron in a Sodium Phosphite Primed Bath Example 3 was repeated except the bath was maintained at 850 degrees Fahrenheit and one-tenth of one percent of sodium phosphite (Na I-IPO .5H O) was employed instead of ferrophosphorus. Similar results were obtained.

Example 11.Descaling Cast Iron in an Elemental Iron Primed Bath Example 3 was repeated except the bath was maintained at 850 degrees Fahrenheit and one-tenth of one percent of elemental iron powder was used instead of ferrophosphorus. Similar results were obtained.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment of the invention is therefore, to be considered as in all respects, illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

I claim:

1. A non-reducing molten alkali metal hydroxide-based salt balt for electrolytic cleaning of metals, said salt bath consisting essentially of the reaction product of at least 50 percent by weight of an alkali metal hydroxide, a minor proportion of a compound selected from the group consisting of alkali metal and alkaline earth metal phosphates, chlorides, carbonates, fluorides, silicates, aluminates, stannates, 'bora-tes, sulfates, sulfites, molybdates, fluosilicates, fluoborates, and mixtures thereof; and a priming amount of at least 0.05 percent of a material selected from the group consisting of elemental phosphorus, finely divided elemental iron, a phosphide, a phosphite, and mixtures thereof.

2. In a process for preparing a non-reducing molten alkali metal hydroxide-based salt bath for the electrolytic cleaning of metals, which bath consists essentially of at least 50 percent by weight of an alkali metal hydroxide, and a minor amount of a compound selected from the group of alkali metal and alkaline earth phosphates, chlorides, carbonates, fluorides, silicates, aluminates, stannates, borates, sulfates, sulfites, molybdates, flllOSillOElltCS, fluoborates, and mixtures thereof; the improvement for priming said salt bath which comprises adding thereto a priming amount of at least 0.05 percent of a material selected from the group consisting of elemental phosphorus, finely divided elemental iron, a phosphide, a phosphite, and mixtures thereof.

3. In a process for electrolytically descaling metal in a non-reducing, non-oxidizing alkali metal hydroxide salt bath consisting essentially of at least percent by weight of an alkali metal hydroxide, and a minor amount of a compound selected from the group of alkali metal and alkaline earth metal phosphates, chlorides, carbonates, fluorides, silicates, aluminates, stannates, borates, sulfates, sulfites, molybdates, fiuosilicates, fluoborates, and mixtures thereof; the improvement which comprises adding thereto a priming amount of at least 0.05 percent of a material selected from the group consisting of elemental phosphorus, finely divided elemental iron, a phosphide, a phosphite, and mixtures thereof.

4. A composition according to claim 1 wherein said alkali metal hydroxide is present in at least eighty percent by Weight.

5. A composition according to claim 4 wherein said alkali metal hydroxide is sodium hydroxide.

6. A composition according to claim 5 where said addit-ive is ferrophosphorus.

7. A composition according to claim 5 wherein said additive is aluminum phosph-ide.

8. A composition according to claim 5 wherein said additive is calcium phosphide.

9. The process of claim 3 wherein the material added is ferrophosphorus.

10. The process of claim 3 wherein the material added is aluminum phosphide.

11. The process of claim 3 wherein the material added is calcium phosphide.

References Cited in the file of this patent UNITED STATES PATENTS 2,133,290 Frazer Oct. 18, 1938 2,134,457 Tainton Oct. 25, 1938 2,261,744 Ostrofsky Nov. 4, 1941 2,395,694 Spence et :al Feb. 26, 1946 2,437,474 Orozco Mar. 9, 1948 2,666,023 Schaaber Jan. 12, 1954 2,738,294 Spence Mar. 13, 1956 2,796,366 Carter June 18, 1957 

2. IN A PROCESS FOR PREPARING A NON-REDUCING MOLTEN ALKALI METAL HYDROXIDE-BASED SALT BATH FOR THE ELETROLTYTIC CLEANING OF METALS, WHICH BATH CONSISTS ESSENTIALLY OF AT LEAST 50 PERCENT BY WEIGHT OF AN ALKALI METAL HYDROXIDE, AND A MINOR AMOUNT OF A COMPOUND SELECTED FROM THE GROUP OF ALKALI METAL AND ALKALINE EARTH PHOSPHATES, CHLORIDES, CARBONATES, FLUORIDES, SILICATES, ALUMINATES, STANNATES, BORATES, SULFATES, SULFITES, MOLYBDATES, FLUOSILICATES, FLUOBORATES, AND MIXTURES THEREOF; THE IMPROVEMENT FFOR PRIMING SAID SALT BATH WHICH COMPRISES ADDING THERETO A PRIMING AMOUNT OF AT LEAST 0.05 PERCENT OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ELEMENTAL PHOSPHOROUS, FINELY DIVIDED ELEMENTAL IRON, A PHOSPHIDE, A PHOSPHITE, AND MIXTURES THEREOF. 